Using three color flow cytometric analysis for the detection of intracellular cytokines, we have studied peripheral blood T cell subpopulations to determine the cytokines produced by individual lymphocytes. Cells from normal individuals and helminth infected patients were magnetically sorted for the CD4+CD27+ and CD4+CD27- subpopulations. Intracellular staining for IL-4, IL-5, and IFN-gamma, subsequent to mitogen stimulation revealed that the CD4+CD27- population included higher frequencies of cells capable of producing IL-5 and not IFN-gamma, IL-4 not IFN-gamma and IFN-gamma and neither IL-4 nor IL-5 compared to the CD4+CD27+ population. Thus, while virtually no CD4+CD27- lymphocytes produce both IL-5 and IFN-gamma, a distinct proportion produced both IL-4 and IFN-gamma (0.1-8.0%) and 66-84% of IL-5-producing cells also produce IL-4. Lymphocytes from normals and patients had the same functional T cell subsets, but the CD4+CD27- lymphocytes from patients had higher frequencies of cells producing IL-4 or IL-5. Normal subjects had higher frequencies of cells producing IFN-gamma. This data demonstrates Th1, Th2, and Th0 cell subsets within the CD4+CD27- lymphocyte population. Studies are underway to examine both peripheral blood and broncho-alveolar lavage-derived lymphocyte populations in allergic individuals with asthma.